Step a Decade Counter using a Parallel Port?

Not sure which decade counter you are using but I think most require that clock input to cycle from high to low before it will add to the counter. Putting a constant 5v signal to the clock will do nothing. Put a switch on the 5v signal and turning the switch on and off should cause your counter to increase if the other pins are set correctly.

Yes, you can use the computer to send out a high/low variable signal. It will require some programing to do it. Just hooking up to the USB on a data line and sending ASCII should give you some movement but you won't have much control. (Assuming your chip is fast enough to read the cycles, some might not be.)

Check the data sheet for the counter you are using. It should tell you what conditions need to exist at each pin for the counter to increment and how long the signal has to be down to work as a count.